Recently, because of a high oil price and a global warming phenomenon based on a large amount CO2 emission, energy is becoming the most important issue in determining the future life of mankind. Even though many renewable energy technologies including wind force, bio-fuels, a hydrogen/fuel cell and the like have been developed, a photovoltaic device using sunlight is in the spotlight in that solar energy, the origin of all energies, is an almost infinite clean energy source.
The sunlight incident on the surface of the earth has an electric power of 120,000 TW. Thus, theoretically, if a photovoltaic device having a photoelectric conversion efficiency of 10% covers only 0.16% of the land surface of the earth, it is possible to generate electric power of 20 TW that is twice as much as the amount of energy globally consumed during one year.
Actually, the world photovoltaic market has explosively grown by almost 40% of an annual growth rate for the last ten years. Now, a bulk-type silicon photovoltaic device occupies a 90% of the photovoltaic device market share. The bulk-type silicon photovoltaic device includes a single-crystalline silicon photovoltaic device and a multi-crystalline or a poly-crystalline silicon photovoltaic device and the like. However, productivity of a solar-grade silicon wafer which is the main material of the photovoltaic device is not able to fill the explosive demand thereof, so the solar-grade silicon wafer is globally in short supply. Therefore, this shortage of the solar-grade silicon wafer is a huge threatening factor in reducing the manufacturing cost of a photovoltaic device.
Contrary to this, a thin-film silicon photovoltaic device based on a hydrogenated amorphous silicon (a-Si:H) allows to reduce a thickness of a silicon layer equal to or less than 1/100 as large as that of a silicon wafer of the bulk-type silicon photovoltaic device. Also, it makes possible to manufacture a large area photovoltaic device at a lower cost.
Meanwhile, a single-junction thin-film silicon photovoltaic device is limited in its achievable performance. Accordingly, a double junction thin-film silicon photovoltaic device or triple junction thin-film silicon photovoltaic device having a plurality of stacked unit cells has been developed, pursuing high stabilized efficiency.
The double junction or triple junction thin-film silicon photovoltaic device is referred to as a tandem-type photovoltaic device. The open circuit voltage of the tandem-type photovoltaic device corresponds to a sum of each unit cell's open circuit voltage. Short circuit current is determined by a minimum value among the short circuit currents of the unit cells.
Regarding the tandem-type photovoltaic device, research is being devoted to an intermediate reflector which is capable of improving efficiency by enhancing internal reflection between the unit cells.